/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    void inorderTraversal(vector<int>& ret, TreeNode* root)
    {
        if (!root)
            return;
        inorderTraversal(ret, root->left);

        ret.push_back(root->val);

        inorderTraversal(ret, root->right);
    }
    vector<int> inorderTraversal(TreeNode* root) {
            // if (!root)
            //     return vector<int>();
            vector<int> ret;
            inorderTraversal(ret, root);

            return ret;
    }
};